Loudspeakers

Abstract

A loudspeaker comprising a panel-form acoustic member adapted for operation as a bending wave radiator and an electrodynamic moving coil transducer having a voice coil mounted to the acoustic member to excite bending wave vibration in the acoustic member. The junction between the voice coil and the acoustic member is of sufficient length in relation to the size of the acoustic member to represent a line drive such that the acoustic member has a mechanical impedance which has a rising trend with bending wave frequency.

Description

TECHNICAL FIELDThe invention relates to bending wave panel loudspeakers, e.g. resonant bending wave panel speakers of the kind exemplified by WO97 / 09842, and to drive motors for such speakers.BACKGROUND ARTIn making electro-dynamic, that is moving coil, vibration transducers for bending wave panel speakers, current thinking on voice coil size and mass tends towards the use of small diameter and low mass voice coil systems, typically of the size of tweeter coils of conventional pistonic speakers. In certain applications, e.g. for driving bending wave panels or diaphragms as exemplified by WO98 / 39947, which are intended to be driven centrally, e.g. so that they can act both pistonically and in bending, such small diameter voice coils may cause power handling and excursion-related problems.For such small diameter voice coils the drive point impedance (Zm) approximates to that of a panel driven at a single point. As the frequency is increased Zm oscillates with modal structure but is on...

AI Technical Summary

Benefits of technology

The portion of the acoustic member circumscribed by the voice coil may be stiffer than a portion of the acoustic member outside the voice coil. The bending stiffness of the acoustic member may be anisotropic. The acoustic member may be curved or dished or otherwise formed to increase its bending stiffness.

The action of the large area voice coil on the diaphragm can produce a distribution of excited modes that results in significant beaming of the radiation on-axis, at least over some of the frequency range. In some applications, such as zoning of the output sound, this may be advantageous, but in many applications off-axis power is desirable. One approach to improving off-axis power is to excite the panel in bending wave vibrations at frequencies near to or greater than the coincidence frequency.

Problems solved by technology

In certain applications, e.g. for driving bending wave panels or diaphragms as exemplified by WO98 / 39947, which are intended to be driven centrally, e.g. so that they can act both pistonically and in bending, such small diameter voice coils may cause power handling and excursion-related problems.

Voice coil diameter has only ever been increased slightly and then only to find that the cell cap, drum-mode resonance becomes dominant and causes premature roll-off.

Other issues that work against low mass voice coils for pistonically driven panels are sensitivity and bandwidth.

But power handling and excursion are still restricted.

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